In aircraft construction, structural components are often connected by connecting elements (rivets), for example stringers and frames, to the sheeting. The compounds produced then take over static and dynamic loads. To date, the metallic connecting elements (rivets) conventionally used in aircraft construction have been protected by complex corrosion protection systems. The protection systems are in this case intended to prevent corrosion both to the structural components and to the connecting elements. Conventionally, the protection systems consist of two, three or four layers which are applied in a plurality of operations, for example by spraying, rolling and/or brushing or electroplating. In the past, the multilayered systems used have often comprised an oxide layer (CAA) or a conversion coating, a primer layer applied thereto and optionally a top coat disposed on the primer layer (wherein a sealing compound can also be provided between the primer layer and top coat).
Drawbacks of the protection systems conventionally used in the past include, in particular, the large number of production process steps, the extent of the solvent immission, the amounts of waste paint and/or the high weight of the coated substrates.
For financial and ecological reasons, these parameters are in need of improvement. Nevertheless, the search for alternative protection systems for aircraft structures which allow reduction of the production costs, solvent immission, the amounts of waste paint and/or weight and comprise, in addition to (assembled) structural components, at least one rivet, is, on account of the high technical requirements placed on protection systems of this type, no straightforward task.
Firstly, they have to ensure an effective barrier against a broad range of agents to which they are exposed. Examples include, in addition to water in its various states, in particular corrosion-promoting electrolytes such as, for example, NaCl solution, aggressive phosphate ester hydraulic fluids such as Skydrol and the like. A closed film formed by a protection system of this type must be an effective diffusion barrier and must be resistant to the agents acting thereon.
Secondly, the adhesion of the protection system to the substrate (for example, structural components having rivets) must be ensured even under mechanical loads. During aircraft maintenance, the coated structural components provided with rivets are stressed as a result of being walked on and machined. During flight, high compressive forces and tensile stresses and also marked fluctuations in temperature often place extreme mechanical loads on the components. These loads are particularly high at interfaces, where different materials having different coefficients of thermal expansion abut one another. In order to resist loads at the respective interfaces, the protection systems have to have high resilience.
A further complication is that the protection systems must adhere effectively not only to a single surface but rather to different surfaces. Connecting elements used in aircraft construction (in particular rivets) are conventionally made of aluminum, titanium or steel, whereas the structural components surrounding them are often painted.
Recent times have seen the development of alternative coating materials based on the sol-gel principle. However, to date, there has been no specific coating material (and no multicomponent system operating in accordance with the sol-gel principle for producing a coating) for aircraft structures comprising, in addition to a structural component, at least one rivet, wherein the coating adheres effectively and reliably to the adjacent different surfaces (rivet, structural component) even under operating conditions while at the same time providing effective corrosion protection.
Reference is made to the following documents (and in some cases brief information is given regarding features that are of relevance in the present invention, but not disclosed in the respective document):
US 2003/0024432 A1 relating to “Corrosion inhibiting sol-gel coatings for metal alloys”.
U.S. Pat. No. 5,175,027 relating to “Ultra-thin, uniform sol-gel coatings”.
U.S. Pat. No. 5,206,285 relating to “Aqueous coating of silane precursor from epoxy and amino trialkoxysilanes”.
U.S. Pat. No. 5,939,197 A relating to a “Sol-gel coated metal”.
U.S. Pat. No. 5,958,578 A relating to a “Hybrid laminate having improved metal-to-resin adhesion”.
U.S. Pat. No. 5,866,652 A relating to “Chromate-free protective coatings”.
U.S. Pat. No. 6,037,060 relating to a “Sol for bonding epoxies to aluminum or titanium alloys”.
DE 198 13 709 A1 relating to a “Process for protecting a metallic substrate from corrosion”.
DE 43 38 361 A1 (corresponding to EP 0 728 164 B1) relating to a “Process for producing compositions based on epoxide group-containing silanes”.
DE 43 03 570 A1 (EP 0 610 831 B1) relating to a “Coating material and process for producing functional coatings”, but not disclosing prepolymers having an epoxy equivalent weight of at least 200.
DE 696 21 941 T2 relating to “Non-chromate corrosion inhibitors for aluminum alloys”.
EP 0 358 338 B1 relating to a “Method and composition for surface treatment”.
EP 1 493 843 A1 relating to a “Coated metallic component”. (Corresponding to WO 2005/003407 A1).
DE 199 52 040 A1 relating to “Substrate with an abrasion-resistant diffusion blocking layer system”, but containing no disclosure regarding the epoxy equivalent weight.
US 2004/0099845 A1 relating to an “Anti-corrosion composition”, but not disclosing mol percentages of components of the respective compositions.
DE 198 57 316 A1 relating to “Powder coated substrates having a top paint based on silanes containing epoxide groups”, but not disclosing mol percentages of all components of the respective paint.
DE 103 20 765 A1 relating to an “Agent for coating metals for corrosion protection”, but not disclosing mol percentages or epoxy equivalent weights.
None of the aforementioned documents discloses a coating material or multicomponent system based on the sol-gel principle for producing a coating, which material or system ensures, after application to an aircraft structural component having at least one rivet, at the same time high resilience, a good barrier effect and effective adhesion to different materials.